Such distance-measuring devices are used to determine, with extreme precision, the distance between a stationary and a moving point in space, usually by an interferometric distance measurement. The moving point in space is usually assigned a measuring retroreflector, which a beam of rays from the optical distance-measuring device is made to follow. Based on the distance information obtained in this manner in conjunction, for example, with further spatial angle information and/or distance information, the position of the moving point in space, i.e., the position of the measuring retroreflector in space is able to be determined.
An optical distance-measuring device is described in European Patent No. 0 919 830. In addition to a light source, it includes an interferometer unit having a measuring retroreflector, a reference retroreflector, a beam-splitter element and a detection unit. The beam of rays arriving from the light source is split via the beam-splitter element into a measuring beam of rays and a reference beam of rays. The measuring beam of rays propagates initially in the direction of the reference retroreflector, is reflected from there in the direction of the measuring retroreflector, and from the measuring retroreflector, is reflected back again in the direction of the beam-splitter element. From the beam-splitter element, the measuring beam of rays propagates in collinear fashion with the reference beam of rays in the direction of the detection unit, via which distance-dependent interference signals are ascertainable. In this device, the measuring beam of rays and the reference beam of rays are able to swivel about the reference retroreflector. This is ensured by a suitably designed swivel unit that includes, inter alia, the light source and several interferometer components. The reference retroreflector for the collimated reference beam of rays, swiveled about a sphere, is realized by the stationary reflecting sphere in conjunction with a focusing lens swiveled together with the reference beam of rays. The collimated reference beam of rays is focused by the lens onto the surface or the center of the reflecting sphere. With the aid of this distance-measuring device, the distance between the stationary reference retroreflector and the measuring retroreflector movable in space is able to be determined interferometrically. By a tracking unit, the measuring beam of rays is made to follow the measuring retroreflector, so that in this manner, a continuous distance measurement is possible, that is, even when the measuring retroreflector is moving in space.
It must be mentioned as a disadvantage of the approach described in European Patent No. 0 919 830 that it is relatively susceptible to dirt. This holds true especially for the variant in which the beam of rays emitted by the light source is focused onto the surface of the metallic, spherical reference retroreflector and is reflected from there. Any dirt existing on the reference retroreflector greatly impairs the interference signal generated. Furthermore, measuring errors and losses with respect to the degree of signal modulation result if the swivel unit is displaced tangentially or radially relative to the ideal position. Therefore, considerable demands are placed on the accuracy of the guidance mechanism of the swivel unit if such problems are to be avoided.
A further optical distance-measuring device is described in Japanese Published Patent Application No. 7-120213. An interferometric distance measurement between a spherical measuring retroreflector and a stationary, spherical reference retroreflector is provided here, as well. The measuring beam of rays and the reference beam of rays are likewise able to swivel about the reference retroreflector. The swivel unit provided for this purpose contains various optical elements such as reflecting prisms and beam splitters for guiding the different beams of rays. The light source is placed outside of the swivel unit. This optical distance-measuring device has the disadvantage that an extremely precise and therefore costly positioning of the various optical elements in the swivel unit is necessary.